The structure of one of the optical components of interest is schematically shown in FIG. 1. Essentially, it is an optically anisotropic film 100 with the optical axis orientation 101 rotating in the plane of the film, the x,y plane in FIG. 1. In simplest realization, the rotation angle α of the optical axis orientation is a linear function of a single coordinate, α=πx/Λ with Λ characterizing the period of the pattern. The thickness L of the film is defined by half-wave phase retardation condition L=λ/n∥−n⊥), where n∥ and n⊥ are the principal values of the refractive indices of the material; and λ is the radiation wavelength. Such a structure imposes a phase shiftΦ=±2α(x,y)on circular polarized beams propagating through it with the sign depending on the handedness of polarization. With account of α=2πx/Λ=qx, where q=2π/Λ, an unpolarized beam is thus diffracted into +/−1st diffraction orders with the magnitude of the diffraction angle equal to λ/Λ. The phase Φ in the equation above, known as geometrical or Pancharatnam phase, does not depend on wavelength, hence the broadband nature of the diffraction. Due to its half-wave plate nature, there are well developed techniques for making the component essentially achromatic in a wide range of wavelengths.
Obtaining large diffraction angles requires that the optical axis modulation period Λ be comparable to the wavelength λ. Liquid crystals (LCs) are the only materials that allow obtaining continuous optical axis modulation patterns at micrometer scale and in a technologically efficient manner. Moreover, due to record high optical anisotropy, Δn=n∥−n⊥˜0.1, the thickness of the film providing 100% diffraction efficiency is also comparable to the wavelength.
The molecules of a LC material are easily aligned along an anisotropy axis of a substrate. There are two major techniques for inducing structural anisotropy on a substrate. In the photoalignment technique demonstrated in FIG. 2, in a first step, the substrate 200 is coated with a material that creates a thin layer 202 (˜10-100 nm) of random distribution of molecules 203. Due to absorption dichroism, the molecules are aligning according to the polarization of typically UV light beam 210, parallel or perpendicular, depending on the so-called photoalignment material, FIG. 2B. Perpendicular aligned molecules 204 is shown in FIG. 2B for certainty. Lastly, the substrate is coated with LC layer 220 the molecules wherein 221 align along the anisotropy axis produced in the photoalignment material 202, FIG. 2C. The LC can be polymerizable for some applications.
The cycloidal polarization modulation pattern is typically obtained holographically in the overlap region of right- and left-circular polarized beams. Holographic technique requires expensive lasers providing coherent beams, optics and opto-mechanical stabilization systems. Radiation power and beam size limitations limit the use of the technique to small components only. The materials used for photoalignment are also expensive, not widely available, and often do not provide strong enough orientation conditions for LC molecules.
Thus, there is a need for a technique that would allow fabricating DWs with the aid of mechanical rubbing of inexpensive polymer films well-developed and commonly used for liquid crystal display technologies. There is a wide prior art related to mechanical rubbing, as for example evident from the U.S. Pat. No. 7,048,619 to Park et al. or U.S. Pat. No. 8,045,130 to Son, et al. However, to the best of our knowledge none addressed the opportunity for producing general patterns with high spatial resolution.